The Locard exchange principle proposes that a person can not enter or leave an area or come in contact with an object, without an exchange of materials. In the case of scent evidence, the suspect leaves his scent in the location of the crime scene itself or on objects found therein. Human scent evidence collected from a crime scene can be evaluated through the use of specially trained canines to determine an association between the evidence and a suspect. To date, there has been limited research directed to the volatile organic compounds (VOCs) comprising human odor, and their usefulness in distinguishing between individuals. In addition, the collection and preservation materials employed for human scent evidence have yet to be evaluated and optimized. The scarcity of scientific background pertaining to human odor and scent collection methods has led to successful legal challenges to the use of canine human scent discriminations for investigative purposes.
The ability of canines to discriminate human scent has been documented as early as 1887. George J. Romanes contributed many fundamental observations as to the ability of dogs to scent discriminate among humans such as the human body leaves an individual odor which a dog can distinguish, individual odors can be determined at great distances and under different environmental stresses, and canines are not deterred from scent discrimination by fragrances. Tracking canines are trained to utilize both human scent and environmental disturbances to locate the track of an individual, but are not given an initial scent to follow. Trailing canines are scented on an object, then asked to determine if the scent of the individual can be detected in an area (uncontrolled environmental conditions) and follow it to the source or until the trail ends. Human scent identification canines are presented with an individual's scent collected from a crime scene, then asked to match the odor from a selection of scents under semi-controlled environmental conditions.
The ability of dogs to match odor collected from different parts of the body has been evaluated. Dutch police dogs were shown to be able to match scent collected from hands to scent collected from the crook of the elbow from the same individual 32% of the time, which is greater than the 16.7% due to chance alone. These dogs also showed the ability to match odor collected from the hands to scent collected from pants pockets of the same individual. Studies in the United Kingdom also have shown that dogs possess the ability to match scent to that of an individual taken from various places of the body with a success rate above 80%, compared to a chance rate of 16.7%.
A study conducted in the United States produced different results. In the evaluation of the ability of canines to match odor collected from different parts of the body, odors were collected using forceps and metal bars. The canines were trained to distinguish between individuals based on hand odor. In the first part of the experiment, the canines were successful in matching a hand odor sample to the correct scent in an array of hand scented objects. In the second part of the experiment, the dogs were required to match hand odor to objects scented by the crook of the elbow by different individuals. The canines were only successful in the identifications 57.9% of the time which was not statistically higher than that due to chance. It was concluded that the inability of the canines to correctly match hand odor and elbow odor brings into question dogs' ability to generalize an individual's odor signature and cross match samples taken from different parts of the body. The lack of controls leading to ambiguous scent matching and appropriate training for the canines used in the U.S. experiment was intentional to evaluate whether canines that have been trained to match odor from a specific part of the body will automatically generalize odors when faced with odors from different areas of the body. Justification for the ambiguity was that a strict interpretation of the individual odor theory would suggest that canines have the ability to automatically generalize odors. The canines did not demonstrate an automatic ability to generalize odors, highlighting the importance of correct training procedures for scent identification canines in order to allow the canine to be able to generalize odors successfully.
Use of canines in identifying a subject has been used with varying success by law enforcement across the globe. What has yet to be addressed is a systematic means of identifying a subject using instrumental analysis, wherein both collection techniques and storage of collection samples are addressed.